Many natural products of pharmacological importance have macrocyclic structures, like the polyketide antibiotic erythromycin, the non-ribosomal peptide cyclosporine or the cryptophycins, a family of depsipeptides and potent antitumor agents. In nature, these macrocyclic compounds are synthesized by modular enzymatic ‘assembly lines’ using polyketide (PK) synthases, non-ribosomal peptide (NRP) synthetases or hybrid NRP/PK synthetases. (See, e.g., Cane, et al., Science, 282:63-68 (1998); Marahiel, et al., Chem. Rev., 97:2651-2674 (1997)). During biosynthesis, the intermediates are bound to the enzymes by a thioester, and, in the final step, are cyclized by an integrated carboxy-terminal thioesterase (TE) domain.
Previous strategies for the synthesis and enzyme catalyzed on-resin cyclization of peptides involved substrates bound via ester or thioester linkage to a solid support. (See, Kohli, et al., Nature, 418:658-661 (2002); Wu, et al., Org. Lett., 5:1749-1752 (2003); Tamaki, et al., Tetrahedron Lett., 47:8475-8478 (2006)). Solid-phase synthesis of linear polyketides that employ diverse reaction conditions have been reported. (See, Umarye, et al., Chem. Eur. J., 13:3305-3319 (2007); Lessmann, et al., Chem. Commun., 3380-3389 (2006); Paterson, et al., Angew. Chem. Int. Ed., 39:3315-3319 (2000)). To facilitate the synthesis of large libraries of macrocyclic compounds, a method that enables direct release and cyclization of compounds on-resin is required.
Cryptophycins, a class of macrocyclic depsipeptides, were first isolated in the 1990s from Nostoc sp. ATCC 53789 and Nostoc sp. GSV 224. (See, Schwartz, et al., J. Ind. Microbiol., 5:113-123 (1990); Golakoti, et al., J. Am. Chem. Soc., 116:4729-4737 (1994); Golakoti, et al., J. Am. Chem. Soc., 117:12030-12049 (1995).) The therapeutic potential of these natural products is derived from their potent and highly selective cytotoxicity, including multi-drug-resistant tumor cell lines. The biological properties generated significant interest in their large-scale isolation, total synthesis and modification. (Smith, et al., Cancer Res., 54:3779-3784 (1994).) Currently, more than 25 naturally occurring cryptophycins, and several hundred synthetic analogs, have been described. Several of these analogs have been identified as advanced anti-cancer therapeutic leads that are being considered for clinical evaluation. (See, Liang, et al., Invest. New Drugs, 23(3):213-24 (2005).) Most natural cryptophycins consist of four hydroxy or amino acids (units A-D, respectively): δ-hydroxy phenyloctenoic acid, 3-chloro-O-methyl-D-tyrosine, (R)-α-methyl-β-alanine (or β-alanine) and L-leucic acid (Scheme 1). (See., e.g., Eissler, et al., Synthesis, 3747-3789 (2006); and Eggen, et al., Med. Res. Rev., 22:85-101 (2002).)

Recently, the gene cluster responsible for production of cryptophycins was characterized from the cyanobacteria Nostoc sp. ATCC 53789 and Nostoc sp. GSV 224. (See Magarvey, et al., ACS Chem. Biol., 1:766-779 (2006).) Furthermore, specific enzymes involved in their biosynthesis have been heterologously expressed, purified and characterized including the cryptophycin thioesterase (Crp TE), which is responsible for the macrolactonization of the linear intermediate. (See, e.g., Magarvey, et al., ACS Chem. Biol., 1:766-779 (2006).)
A need exists to prepare macrocyclic compounds, like cryptophycins, in a manner that employs the benefits of solid support chemistry (e.g., adaptability and easy modification) and limits undesired side reactions, such as elimination by-products. This disclosure describes the solid-phase synthesis and on-resin cyclization of crytophycin analogs and general methods for forming macrocycles using solid support chemistry.